The standard needle used for subcutaneous or intramuscular injections is in the form of a hollow shaft provided with a sharpened open end. This type of needle has been found to be unreliable in ensuring accurate delivery rates, in particular when used in conjunction with an infusion pump or some or other delivery device which depends for delivery on the magnitude of pressure applied to the liquid being delivered.
The lack of accuracy is thought to be due to the build up of pressure at the delivery point which opposes the driving pressure applied to the liquid.
Other have tried to combat the problem by designing needles which were intended to avoid the problem of pressure build up arising from the orifice becoming plugged by tissue on entry into the skin.
U.S. Pat. No. 4,413,993 (Guttman), U.S. Pat. No. 4,790,830 (Hamacher), and U.S. Pat. No. 4,838,877 (Massau) each disclose a hypodermic or intravenous delivery needle having one or more apertures located on the side of the shaft removed from the sharpened tip.
U.S. Pat. No. 2,748,769 (Huber) discloses a hypodermic needle having a curved or bent tip cut in a plane that extends along the side of the needle towards which the bend is made and thereby providing an orifice which is not plugged by tissue upon insertion into a subject, the curved surface being provided with an auxiliary delivery orifice which ensures delivery when the main orifice rests against a vein wall.
U.S. Pat. No. 3,076,457 (Copen) discloses a hypodermic needle having an aperture at the tip and also having an opening which extends along the side of the shaft for part of its length.
None of these needles provides a satisfactory solution to the problem of ensuring accurate delivery using a delivery device in which the delivery rate depends on the generation of pressure, because in each case, the problem which has been addressed has been the provision of an additional orifice which will not become plugged upon insertion or which will not rest against a vein wall. While these problems must be addressed in designing a needle for use with an automated drug delivery device, it has been found that these factors alone do not account for the major variations in delivery rate which occur when using a device which depends for delivery on pressurising a supply of drug.
It has been found, when conducting tests of devices which operate on the pressurised reservoir principle, that there are large variations in the resistance to delivery into the skin not only when comparing different subjects or different delivery sites on the same subject (which could be accounted for by differences in tissue composition at the delivery site), but also when the resistance to delivery is monitored over time during a single test at a single site.
If the resistance to delivery (i.e. the impedance to the flow of drug) varies while the pressure effecting delivery remains fixed, then clearly the delivery rate (which is dependent on the difference in pressure between the reservoir and the delivery site) will fluctuate. Clearly this has serious implications for the delivery accuracy of such devices.
The best results in subcutaneous delivery tests of this type are obtained when using a needle such as that disclosed by Huber or by Copen (see above), wherein the aperture has a greater external surface area than internal surface area. Without wishing to be bound by any theoretical explanation, it is conjectured that the major obstacle to ensuring accurate delivery rates is not that the orifice becomes plugged, but rather that the difficulty arises from poor absorption by the tissue adjacent to the orifice(s), and/or from variations in the structure of the tissue at the orifice. This means that the drug accumulates at the orifice and the pressure builds up until absorption commences, since a given surface area of tissue can only absorb a certain amount of drug at a given pressure. In cases where the external aperture surface area is increased relative to the internal aperture surface area, the absorption (by a larger tissue area) is correspondingly better and fluctuations are reduced.
Nevertheless, it is not practicable to replace conventional delivery needles with needles of the type disclosed by Huber or Copen because of the substantial difference in manufacturing cost involved. A conventional delivery needle can be formed by cutting hollow steel tubing to the correct length, and grinding or in some other way machining the cut end to provide the necessary sharpness for skin penetration. The thus formed tip is of course automatically provided with a delivery orifice, and the needle is inexpensive.
In order to manufacture a needle of the type disclosed in any of the prior art documents referred to above, the initial steps of creating a needle must first be followed, and then the additional holes or orifices must be machined into the side of the needle. In the case of small diameter needles such as needles having an external diameter of less than 0.7 mm and an internal diameter of less than 0.45 mm (approximately 22 gauge or higher), this is a relatively complicated task when compared to the preceding steps.
The orifice may have to be created by laser drilling a hole at the correct point, by pressing a revolving disc cutter against the curved surface of the Huber needle, or by removing a quarter-cylinder length from the side of the shaft in the case of the Copen needle (no indication is given by Copen as to how one would go about creating the type of aperture disclosed). In all cases, the operations require careful positioning of the needle relative to the machining means, and a delicate machining of each and every needle. The magnitude of the cost differential between such needles and conventional needles arises from the differences between the complexity of the manufacturing processes involved in each type of needle.
It is an object of the present invention to provide a needle which has improved delivery characteristics when connected to an infusion system, and which can be manufactured cheaply and easily in large numbers at the same time unlike the prior art needles referred to above.